The float glass process is well known for making sheets of glass. In a typical float glass process, batch materials are heated to form molten glass. The molten glass is then poured onto a bath of molten tin. The molten glass is drawn along the bath of molten tin and simultaneously cooled and attenuated to form a dimensionally stable continuous sheet of glass, typically referred to as a glass ribbon. The sheet is then removed from the bath for further processing.
Two types of furnaces are used in the float glass process, an air-fuel furnace and an oxy-fuel furnace. In an air-fuel furnace, fuel is mixed with warm air and combusted to provide heat to melt the glass batch materials.
In an oxy-fuel furnace, oxygen, not air, supports combustion. As a result, an oxy-fuel furnace provides a much more efficient melt than an air-fuel furnace because energy is no longer being wasted heating up nitrogen in the air and oxy-fuel flames have a higher flame temperature that radiates more efficiently. The increased melting efficiency allows more tonnage to be processed through an oxy-fuel furnace than through a similarly sized air-fuel furnace.
Both air-fuel and oxy-fuel furnaces have water in their atmospheres. The head space (the area of the furnace above the molten glass) in an oxy-fuel furnace has a higher concentration of water than in an air-fuel furnace because the oxy-fuel atmosphere lacks the nitrogen provided in an air-fuel furnace that dilutes the total water formed by combustion. Stoichiometrically, the water typically constitutes about 66% by volume of the head space in an oxy-fuel furnace versus 18% in an air-fuel furnace. Since the amount of water in the glass melt is proportional to the square root of the concentration of water in the head space, glass melted in an oxy-fuel furnace has a 1.7 to 2 times higher water concentration than glass melted in a conventional air-fuel furnace. Typically, glass melted in an oxy-fuel furnace contains more than 0.045 weight percent water based on the total weight of the composition.
At the stage of the float glass process where molten glass is poured onto molten tin, the molten tin temperature in the float bath ranges from 1800° F. to 1900° F. (981° C. to 1037° C.). At 1800° F., at the glass-tin interface, water that diffuses out of the molten glass dissociates into hydrogen and oxygen. Because hydrogen is not very soluble in tin at 1800° F., much of the hydrogen does not dissolve in the tin but remains in the atmosphere of the bath. Some of the hydrogen from the disassociation of water gets trapped at the interface between the molten glass and tin and ultimately impinges on the bottom surface of the glass ribbon and form defects along the ribbon surface typically referred to as open bottom bubbles. The open bottom bubbles can be described as voids in glass that generally have an inverted U-shaped cross section. The presence of open bottom bubbles increases the overall defect density of the glass.
Customers set requirements for the defect density of glass for certain applications. The standards are very difficult to meet with conventional float glass processes due to the presence of open bottom bubbles.
The present invention provides a novel apparatus and method that yields float glass having a lower total defect density as a result of reduced open bottom bubble defects.